1. Field of the Invention
The present invention relates to a curable organopolysiloxane composition, an optical device sealing material comprised of the composition, and an optical device which is sealed by the cured said composition.
2. Description of the Related Art
An addition-curable type organopolysiloxane composition contains an organopolysiloxane which contains an aliphatic unsaturated group such as an alkenyl group and an organohydrogen polysiloxane; and thus, it gives a cured product by curing with a hydrosilylation reaction. A cured product obtained as mentioned above not only has excellent heat resistance, cold resistance, and electrical insulation properties but also is transparent; and thus, it is used in many optical uses such as an LED sealing material.
In a polysiloxane used in an optical field, high transparency and high refractive index are required so that, to meet these requirements, a method in which dimethylsiloxane-diphenylsiloxane copolymer or polymethyl phenylsiloxane is used for main skeleton is generally used. However, it is difficult to obtain a cured product having refractive index of 1.54 or higher when these linear polysiloxanes are used. On the other hand, a cured product of a polysiloxane which is made branched and introduced with phenyl groups can give refractive index of about 1.53 to 1.54; but the cured product obtained thereof is not elastic but hard resin-like.
Accordingly, an example is reported to use a homopolymer of diphenyl siloxane in its main skeleton as a linear polysiloxane which gives a cured product having refractive index of 1.54 or higher. However, it is difficult to synthesize a homopolymer of diphenyl siloxane having a long chain, so that many cured products using these polysiloxanes are synthesized from a relatively short chain material.
Meanwhile, there may be mentioned a resistance to a thermal impact, i.e., a thermal shock resistance, as one necessary reliability in an optical use, especially as an sealing material for an LED. Peeling-off from a package or a crack of a cured product by a thermal shock is caused not only by a thermal stress due to temperature change but also by an internal stress of the cured product such as a residual stress generated during a curing process. Generally, a cured product formed from a short chain material has high crosslinking density, so that it is difficult to relax the stresses as mentioned above, thereby decreasing its thermal shock resistance. Therefore, also in a cured product using a homopolymer of diphenyl siloxane whereby having refractive index of 1.54 or higher as mentioned above, there have been such problems as peeling-off from a package and a crack of a cured product by a thermal shock test (Patent Document 1 to Patent Document 7).